1. Field of the Invention
The present invention relates to a start ID recording system for use in a digital audio information recording apparatus, and in particular to a start ID recording system for recording a start ID signal identifying a starting head of a digital audio signal such as a music signal, wherein the start ID signal is recorded on a recording medium at an appropriate position coincident with the starting head of the digital audio signal for realizing a high speed search thereof without an omission of a head portion of the digital audio signal when the digital audio signal is reproduced.
2. Description of the Prior Art
In recent years, a digital compact disc (DCD), mini disc (MD) and other similar recording mediums, as well as a digital audio tape recorder (DAT), have been introduced and put into practical use in various conventional digital audio recording systems. In such a conventional digital audio recording system, a start ID signal such as a specified code number or a head signal for identifying a starting head of a digital audio signal is recorded at a start head position of the digital audio signal so that a desired audio signal can be quickly searched by identifying the start ID signal in a reproduction operation thereof.
However, it has been difficult to precisely record a start ID signal at a precise start head position of a digital audio signal such as a music signal because of the reasons discussed next.
Namely, in a conventional digital audio recording system where a start ID is automatically recorded by detecting a level of an input digital audio signal, when a level higher than a predetermined threshold level is detected, it is determined that an audio signal such as a music signal is started and then a start ID signal is recorded at this point. However, in this type recording system, the predetermined threshold level should be set up sufficiently higher than an audio signal absent level (referred to as "silent level", hereinafter) in order to avoid inclusion of a noise component in detecting the audio signal level. Moreover, it is essentially required to integrate an input digital audio signal in order to remove a pulse noise component.
In consideration of these matters mentioned above, in order to detect the predetermined threshold level, it is indispensable that an inherent time delay (t.sub.D) is caused in a degree of zero point several seconds or more from the actual starting time of the input audio signal to the time of detecting the predetermined threshold level. Therefore, if the music signal is searched and reproduced at the delayed time of detecting the start ID, there has been a problem that a rising head portion of the desired music signal located before detecting the start ID is omitted from reproduction.
In order to eliminate this problem, another conventional digital audio recording system has been disclosed as shown in FIG. 11. In this conventional recording system, an input digital audio signal (As) is delayed by a predetermined delay time T.sub.D by means of a known delay unit to thereby obtain a delayed audio signal (As') after the predetermined delay time T.sub.D from the actual starting time of the input audio signal. The predetermined delay time T.sub.D for audio signal is set up to be equal to or slightly larger than the inherent delay time (t.sub.D) for generating a start ID at the time of detecting the predetermined threshold level (Ls).
However, in this conventional system, when the recording system is used in a moving condition, for example, for use as a portable apparatus or in a vehicle, erroneous effects of tracking jumps of a scanning optic and the like are occasionally caused due to shocks or vibrations and also high level noise components are possibly detected beyond a predetermined threshold level (Ls). Then, the noise components are erroneously detected and are confused with a rising portion of an actual audio signal level even though the predetermined threshold detection level (Ls) is set up high as shown in FIG. 12. As the result of this, a starting ID such as a music number is erroneously renewed.
Moreover, in order to avoid an erroneous operation due to high noise components, if the threshold level is set up excessively higher than the silent level (L.sub.O), when in a reproduction mode for reproducing, for example, classical music having a long lowest weak level in a beginning portion thereof and having a gradually higher portion thereof in level as shown in FIG. 13, the inherent delay time (t.sub.D) for a start ID generation becomes considerably large in length. Therefore, when the predetermined delay time T.sub.D for delaying the audio signal is set up to be suitable for general music such as popular music and the like, the predetermined delay time T.sub.D for audio signal is in some cases smaller in length than the inherent delay time t.sub.D for start ID generation. As a result, when the start ID is recorded after the inherent time delay t.sub.D from the actual starting time of the input audio signal and the delayed audio signal As' is generated after the predetermined time delay T.sub.D (which is smaller than t.sub.D) from the actual starting time of the input audio signal, a problem arises in that an omission of a head portion of an input audio signal such as in a classical music signal is undesirably caused in the reproduction operation mode thereof.